# package ocaml-base-compiler

# Parametric diffing

This module implements diffing over lists of arbitrary content. It is parameterized by

- The content of the two lists
- The equality witness when an element is kept
- The diffing witness when an element is changed

Diffing is extended to maintain state depending on the computed changes while walking through the two lists.

The underlying algorithm is a modified Wagner-Fischer algorithm (see <https://en.wikipedia.org/wiki/Wagner%E2%80%93Fischer_algorithm>).

We provide the following guarantee: Given two lists `l`

and `r`

, if different patches result in different states, we say that the state diverges.

- We always return the optimal patch on prefixes of
`l`

and`r`

on which state does not diverge. - Otherwise, we return a correct but non-optimal patch where subpatches with no divergent states are optimal for the given initial state.

More precisely, the optimality of Wagner-Fischer depends on the property that the edit-distance between a k-prefix of the left input and a l-prefix of the right input d(k,l) satisfies

d(k,l) = min ( del_cost + d(k-1,l), insert_cost + d(k,l-1), change_cost + d(k-1,l-1) )

Under this hypothesis, it is optimal to choose greedily the state of the minimal patch transforming the left k-prefix into the right l-prefix as a representative of the states of all possible patches transforming the left k-prefix into the right l-prefix.

If this property is not satisfied, we can still choose greedily a representative state. However, the computed patch is no more guaranteed to be globally optimal. Nevertheless, it is still a correct patch, which is even optimal among all explored patches.

`type ('l, 'r, 'eq, 'diff) patch = ('l, 'r, 'eq, 'diff) change list`

A patch is an ordered list of changes.

```
val diff :
weight:(('l, 'r, 'eq, 'diff) change -> int) ->
test:('state -> 'l -> 'r -> ('eq, 'diff) result) ->
update:(('l, 'r, 'eq, 'diff) change -> 'state -> 'state) ->
'state ->
'l array ->
'r array ->
('l, 'r, 'eq, 'diff) patch
```

`diff ~weight ~test ~update state l r`

computes the diff between `l`

and `r`

, using the initial state `state`

.

`test st xl xr`

tests if the elements`xl`

and`xr`

are compatible (`Ok`

) or not (`Error`

).`weight ch`

returns the weight of the change`ch`

. Used to find the smallest patch.`update ch st`

returns the new state after applying a change.

## Variadic diffing

Variadic diffing allows to expand the lists being diffed during diffing.

```
val variadic_diff :
weight:(('l, 'r, 'eq, 'diff) change -> int) ->
test:('state -> 'l -> 'r -> ('eq, 'diff) result) ->
update:('l, 'r, 'eq, 'diff, 'state) update ->
'state ->
'l array ->
'r array ->
('l, 'r, 'eq, 'diff) patch
```

`variadic_diff ~weight ~test ~update state l r`

behaves as `diff`

with the following difference:

`update`

must now be an`update`

which indicates in which direction the expansion takes place.